I experienced similar happenings while growing up. On several occasions, always while lying supine, i.e. head aimed at the ceiling, and usually just before nodding off, I'd experience an elevation of awareness. My eyes would track over to some corner where the wall meets the ceiling. I would percieve a blackish cloud, and would follow it as it drifted to a position right over my head. Then it would descend and hover over me.

This was all very interesting, except that I would be almost totally paralyzed at this point: the brain, in order to prevent somnambulence, normally stops routing requests to voluntary muscles just before sleep; and so, there I was, not really knowing what the hell was going on, there in the dark, with some kind of dark entity hovering above my person.

The emotional state I experienced was a slowly rising wave of panic and then terror. At that point, the sheer will to move takes over: at some basic, instinctive level, you know that if you can just move, the whole situation will just dissolve. People in this state usually report trying to activate every muscle they can think of in series. I'd manage to move a little bit, maybe make a weak vocalization. This would go on for a few seconds, until the barrier would break down, and I'd be able to move again.

At that point, I'd sit bolt upright, and reality would come flooding in. There I'd be, in my room, everything normal again.

This became a great incentive to sleep on my stomach, as this only happened while sleeping on my back. :)

The phenomenon, or what few instances of it there were anyway, ended when I was in my late teens. The last time it hapened was actually because I wanted to: I deliberately recreated the conditions of a previous night-terror in my mind, and sure enough, relapse! This time, it was not terrifying, just intensely odd.

The only similar experiences I've had since then have been while using Unisom to try to get some extra sleep. Almost without variation, I'll wake up (only partially!) after a period of restless sleep. Instead of the black cloud forming in a corner and floating down over me, I'd get a sensation more like I was just surrounded by evil nastiness. The last time it happened, I happened to have a hand on my chest, and could feel my heart racing at what felt like about 210bpm. (I know this because it usually only gets to about 180 during normal exercise, and I know how that feels.) I would usually have a feeling of rapid pulse during these occurrences, and that explains it. The most prominent perception would be of the blood rushing rapidly in my ears - that was the scariest part. One reason not to use Unisom, I guess. Again, I would just try to thrash around a bit until I shook the paralysis off. Unlike the "normal" occurrences, Unisom-induced night terrors tend to leave me with a distinctly nasty aftertaste that doesn't go away until I drift back off to sleep.

If you're trying to get some sleep, try exercising instead. Depending on chemicals to make you go to sleep is bad.